Add script to append required DHCP options to the Netplan config

The Netplan config is created by cloud-init, but it needs to be
slightly modified and reapplied.  This script takes care of the
modification.

See these issues for more details:
- cisagov/skeleton-packer#300
- canonical/cloud-init#4764

.pre-commit-config.yaml

@@ -130,6 +130,8 @@ repos:
     rev: v1.5.1
     hooks:
       - id: mypy
+        additional_dependencies:
+          - types-pyyaml
   - repo: https://github.com/asottile/pyupgrade
     rev: v3.10.1
     hooks:

cloud-init/fix-dhcp.tpl.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+
+"""Append the necessary DHCP options to the Netplan configuration.
+
+The Netplan configuration is created by cloud-init, but it needs to be
+slightly modified and reapplied.  This script takes care of the
+modification.
+
+See these issues for more details:
+- cisagov/skeleton-packer#300
+- canonical/cloud-init#4764
+
+This file is a template.  It should be processed by Terraform.
+
+"""
+
+# Third-Party Libraries
+import yaml
+
+# Inputs from Terraform
+NETPLAN_CONFIG = "${netplan_config}"
+
+with open(NETPLAN_CONFIG) as f:
+    # Load the current Netplan configuration
+    config = yaml.safe_load(f)
+    # Add a dhcp4-overrides section to each network
+    config["network"]["ethernets"] = {
+        k: v | {"dhcp4-overrides": {"use-domains": True}}
+        for (k, v) in config["network"]["ethernets"].items()
+    }
+
+# Write the results back out to the Netplan configuration file
+with open(NETPLAN_CONFIG, "w") as f:
+    f.write(yaml.dump(config))